Numerous polyethylene compositions for the production of pipe are known. Pipe materials are classified such as PE80 or PE100. The service temperature for PE100 is 20° C. The ISO9080 classification guarantees that a PE100 material will have a lifetime of at least 50 years at 20° C. using internal stress of 10 MPa.
JP-11-058607 A2 discloses a steel pipe coated with a polyolefin which shows good low temperature impact resistance at −60° C. In order to achieve these properties, this Japanese patent application proposes the use of a polyethylene resin having a density of from 0.915 to 0.935 g/cm3.
EP 0679704 A1 also deals with improving the impact resistance at low temperatures, such as −45° C. or lower. In order to achieve this aim, this European patent application suggests the use of a blend of high-pressure low-density polyethylene having a density of between 0.915 to 0.930 g/cm3 with an ethylene-olefin copolymer having a density of 0.895 to 0.920 g/cm3.
JP-11-106682 A2 and JP-09-143400 A2 both disclose resin compositions suitable for powder coating, comprising a blend of ethylene polymers, including acid modified ethylene polymers, polyethylenes of various densities and elastomeric components.
EP 1555292 A1 discloses a polymer composition suitable for extrusion coating, for example for preparing multilayered materials, wherein the composition comprises a multimodal high-density polyethylene and a low-density polyethylene.
EP2072587A1 deals with coated pipes having a layer of multimodal polyethylene. The multimodal ethylene copolymer is a copolymer of ethylene with one or more alpha-olefin comonomers having from 4 to 10 carbon atoms and has a weight average molecular weight of from 70000 g/mol to 250000 g/mol and a melt index MFR2 of from 0.05 g/10 min to 5 g/10 min, a melt index MFR5 of from 0.5 to 10 g/10 min and a density of from 945 kg/m3 to 958 kg/m3. The coatings have a high stiffness, good properties at elevated temperatures and acceptable stress-cracking properties.
In several applications such as tanks, drainage systems, soil and waste pipe systems the service temperature is at much lower temperatures. At low temperatures of lower than 0° C., such as −45° C., polyethylene pipes and pipe coatings tend to become brittle.
Especially for steel pipe coating applications installed in low temperature regions like Russia an elongation at break at low temperatures of −45° C. is an important parameter for transport, storage and installation of the coated steel pipes. However, elongation at break decreases with increasing polymer density which is desirable for improving other parameters such as abrasion resistance, stiffness and crystallization speed. Density of a polyethylene resin can be directly influenced by its comonomer content.
WO 2007/141022 discloses a polyethylene composition for steel pipe top coatings which show a good elongation at break at low temperatures of −45° C. The base resins of these compositions, however, have a rather low density of not more than 940 kg/m3.
There is still a need for polyethylene compositions for pipe and pipe coating applications, especially steel pipe top coatings, which have a high low temperature elongation at break at a low comonomer content and thus at a high density of the base resin.